The present invention relates to a method for identifying the type of an input video signal, namely, whether the input signal is a video signal of a standard television signal picked up by a television camera or a telecine-converted video signal generated from a motion picture film by the 2-3 pull-down method, and also to a method for converting the input video signal formed in accordance with the interlaced scanning into a video signal similar to a sequential scanning signal.
The video signals for standard television system such as the NTSC system include a video signal generated from a film of a movie. Such a movie film video signal comprises 24 frames per second. On the other hand, the video signal for NTSC system is an interlaced video signal comprising 30 frames, or 60 fields, per second. Hence the movie film video signal must be converted to the standard television signal by the 2-3 pull-down method.
In the 2-3 pull-down method, the film is scanned to generate a video signal sequentially scanned at the frequency of 24 Hz. For example, the first frame of the film is converted to correspond to first and second fields, the second frame to third to fifth fields, the third frame to sixth and seventh fields, and the fourth frame to eighth to tenth fields.
When one frame of the film is converted to three fields, such as in the case of the second and fourth film frames, the last field has the same content as the first field. Namely, the fifth and tenth fields coincides with the third and eighth fields, respectively. Hence the two frames of the movie film correspond to five fields of the video signal for the standard television system, wherein the frames are alternately repeated twice and three times.
In the thus generated video signal, the third field and the fifth field, and the eighth field and the tenth field become identical. Hence the difference between the second and third frames, and the difference between fifth and sixth frames become zero. Such a relationship occurs every five fields.
Namely, in order to determine whether the television video signal is a telecine-converted signal produced from a film or originally a television video signal, the absolute values of the differences between the frames are added up during a period corresponding to one field. The added value is compared with a predetermined threshold and when the added value is larger than the threshold, the field is determined as a moving field. On the other hand, when the added value is smaller, the field is a still field. The still field is generated every fifth field in the video signal produced from the film in the case of the 2-3 pull-down system, forming a periodic pattern. Hence the telecine-converted video signal can be discriminated from the standard television video signal if the period pattern in which the still field appears every five fields is detected.
However, in the above described detecting method, since the added absolute values between the fields must be compared over a plurality of fields, for example, at least five fields in the case of 2-3 pull-down method, it is difficult to quickly respond to the change of the video signals. In addition, the film may be edited so that the periodic pattern of the still field in the telecine-converted video signal may be disturbed. Hence the telecine-converted video signal cannot be quickly determined.
Moreover, if another method besides the 2-3 pull-down method is used, the detecting method must be modified in accordance with the periodic pattern.
Another problem which occurs in the telecine-converted video signal is that since two fields constitute one television frame, some television frame comprises fields produced from different film frame. For example, the third television frame, which comprises the fifth field generated from the second film frame and the sixth field generated from the third film frame. The image of such a frame is inferior in quality to that of the original film frame.